This invention is directed to a device for measuring the density of liquid and gaseous substances with a plate, designed to be introduced into the substance to be measured, a holder holding the plate, an electronic unit, at least one vibration exciter connected to the unit and operatively associated with the plate, and at least one vibration detector also connected with the electronic unit and operatively associated with the plate.
In many processes, it is necessary to measure the density of liquid or gaseous substances either continuously or quasi-continuously. It is known that the resonant frequency of a vibrating solid body, which is surrounded by a liquid or a gas, depends on the density of the liquid or gas. Consequently, it is possible to reduce a density measurement to a period measurement or a frequency measurement.
Various devices for measuring the density in accordance with this principle are already known, and the vibrating bodies usually comprise tuning forks or tubular metal bodies. Because of their relatively complicated form, however, such bodies can frequently not be produced with sufficient precision, or can be produced only at great expense. This is particularly the case because the so-called thermocompensating alloys, whose modulus of elasticity is substantially temperature-independent within certain limits, are relatively difficult to process. In addition, the resonant frequencies of these vibration bodies with complicated forms depend not only on the density but, in a manner difficult to control, also on other parameters, particularly the temperature and the holding arrangement. Frequently, it is difficult to mount these complicated vibrating bodies so that the holder does not influence the resonant frequency and does not effect an additional temperature dependence.
An already known device uses a plane circular plate as a vibrating body, and this plate is so excited, at its center, that circular nodal lines are formed during its excitation. The plate is held by a holder, which has three pairs of holding points. The three pairs of points clamp the plate at three holding points or areas arranged on the circular nodal line, these holding points or areas being displaced from each other by an angle of 120.degree..
However, this known device has only a low measuring accuracy. Since the amplitude of the natural vibration of a free plate increases relatively rapidly inside and outside the circular nodal line, the vibration is influenced, to a relatively great extent, by the three pairs of points, and particularly by the cross-section and surface pressure of the points, so that the measurement can be falsified. This is even more the case, as it is rather difficult in practice to adjust the plate so that the three pairs of points act exactly at the nodal line. Furthermore, material expansions or contractions, caused by temperature variations, can likewise cause a displacement of the nodal line with respect to the pairs of holding points. Besides, this type of holding a plate is not very suitable for rough operation in industrial use.